Companies That Amass Vast Amounts of Personal Data Should Learn From Agency's Mistakes, He Says

SAN FRANCISCO—To a degree shared by few, John C. Inglis knows the risks of collecting a lot of data on people.

Until January, Mr. Inglis, who goes by "Chris," was the number-two official at the National Security Agency. He spent much of 2013 pushing back against disclosures from former contractor Edward Snowden about the extent of NSA surveillance.

Now, the former NSA deputy director is warning technology companies...